PROJECT SUMMARY/ABSTRACT: STRATEGIES TO IMPROVE OUTCOMES FOR TNBC PATIENTS INTEGRATING SUBTYPE-SPECIFIC GENOMIC AND IMMUNE-BASED DISCOVERIES Triple-negative breast cancer (TNBC) is one of the most aggressive subtypes of breast cancer, representing ~25% of all breast cancer deaths due to lack of effective therapies. To address this challenge, our team integrated expertise in molecular genetics, oncology and bioinformatics, and 1) classified TNBC into distinct molecular subtypes with unique signaling, biologic and immune features, and 2) discovered and validated novel gene rearrangements in oncogenic and immune-regulatory genes that provide a diverse array of mechanisms by which TNBC tumors evade immune surveillance. Many tumor cells co-opt innate and adaptive immune system mechanisms through direct upregulation of PD-L1 or through recruitment of immune suppressive cells. These forms of immune evasion are in play for tumors with high gene mutation burdens, such as select TNBC subtypes. In the current period, we discovered that some TNBC tumors with high mutation loads remain ?immune cold? due to absence of immune cells in the tumor and enrichment of immune suppressive cells in the microenvironment. Immunotherapies targeting the PD-1/PD-L1 pathway continue to be a paradigm shift in the treatment of metastatic cancers with high mutation rates, thus it is critical to understand how to align appropriate TNBC patients with this therapeutic advance. As combination chemo-immunotherapy advances toward FDA approval for metastatic TNBC, it further catalyzes the need to understand how specific chemotherapeutic agents can enhance TNBC subtype-specific response to anti-PD-1/PD-L1 therapy in order to determine the most efficacious combinations. The mechanisms by which chemotherapy modulates neoantigen burden, T-cell function, and immune-suppressive cell function in TNBC are largely unknown. We propose to test the interrelated hypotheses that: (1) standard chemotherapies modulate tumor immune response targeted by anti-PD-L1 therapy in a TNBC-subtype specific manner; and (2) subtype-specific TNBC preclinical models will provide insights to chemotherapy-specific systemic and tumor microenvironment mechanisms of immune suppression. The project's Specific Aims are: Aim 1: To determine mechanisms by which standard chemotherapies modulate tumor immune response in combination with checkpoint immunotherapy; Aim 2: To determine mechanisms by which by tumor secreted proteins and exosomes modulate the immune system in metastatic TNBC subtypes; and Aim 3: To evaluate the feasibility of targeting the tumor microenvironment myeloid-derived cells. IMPACT: Our overall goal is improved outcomes for TNBC patients through alignment of combination therapy directed by knowledge of TNBC subtypes and immunomodulatory features. Through the comprehensive analysis of biospecimens, our established multi- disciplinary team will develop biomarkers for PD-L1 response and define the effects of chemotherapy on distinct immune cell populations. The discovery of new immune-evasion mechanisms will lead to actionable molecular targets for future drug discovery efforts impacting metastatic TNBC and other breast cancers.